Medical devices with limited data storage and memory capacity are well known in the art. Two common examples of such devices are hearing aids and pacemakers. Pacemakers or other such implantable pulse generators (IPGs) in particular have requirements for storage and transfer of data that sometimes exceeds the storage and memory capacity available. Some IPGs include means for storing data related to cardiac events such as episodes of spontaneous heart rate that are higher or lower than an acceptable or previously established rate. Stored data related to one or more cardiac events are useful in assessing the functioning of the IPG and in monitoring the progress of the patient.
Digital signal processing (DSP) has proved to be a useful tool in the environment of implantable medical devices such as implantable pulse generators. Using DSP technology, an incoming sensed heart signal may be converted to a digital signal, e.g., an 8-bit signal. This conversion may occur at a predetermined sample rate. For example, episodes of Intracardiac Electro Cardiogram (IEGM) may be processed using DSP. The IEGM is one type of signal in which heart contractions may be identified.
Typically an input signal from an IPG is amplified. The signal may then be converted to a digital signal (using, for example, A/D, or analog to digital converters). Then the signal may be digitally processed, generally by filtering the resulting digital data streams. The result from this process is generally a number of digital data streams. Each data stream is more or less a digitally processed representation of an IPG input signal. Based upon the information in these streams, DSP technology may be used to determine heart contractions. As stated above, a physician may use information about these contractions to assess and monitor the efficacy of IPG therapy.
Typically, data is collected continuously while the patient is using the IPG. A physician is only able to view the data when the patient and the IPG are available for evaluation, e.g. when the patient is in the physician's office. At that time, the IPG may be linked to an interrogation device with a display, which shows the data being collected at the time the patient is being examined.
However, the most interesting episodes of IEGM generally occur when the patient is proceeding about his normal business away from the physician's office. Thus, some IPGs (and other implantable therapeutic devices) have the capability to store data, such as an IEGM, for later viewing by the physician. At the time of viewing, the IPG may be linked to an interrogation device with a display that communicates the stored data. Because implantable devices are, of necessity, small enough for implantation in a human body, their available storage space is limited. Thus, the data, such as a digital IEGM, needs to be compressed as much as possible without losing the sense of the original signal.
In a typical compression method, more than one data stream may be received and/or processed (e.g. transferred, transmitted, compressed or stored, etc.) at a given time. Sometimes, the data streams may arrive at different rates. Storage of more than one data stream, particularly if the streams arrive at different rates may require significant amounts of memory. For example, two data streams may start out with the same fixed width (e.g. each sample may be 8 bits wide) and a fixed sample rate, which is the number of signal (sample) values being received or processed per unit of time, (e.g. each sample may be transmitted at a sample rate of 200 transmit units per second). However, after the data streams are compressed, the width and sample rate of the two streams may differ.
For example, data stream 1 and data stream 2 may both be 8 bits wide prior to compression. However, after compression, one data value of data stream 1 may be reduced to a single bit whereas one data value of data stream 2 has been reduced to 5 bits.
After compression, the streams may be combined into one 16-bit word, and transferred to Random Access Memory (RAM). This may be accomplished using Direct Memory Access (DMA) which transfers the data to RAM without using a microprocessor or Central Processing Unit (CPU). A DMA unit may be programmed to transfer a fixed amount of data (for example, the 16-bit word described above) from a data source to a destination, such as RAM. Thus, DMA transfer occurs at a fixed rate (in the above case, 16 bits per unit of time.) However, the data arriving to be transferred via DMA from data stream 1 and data stream 2 continues to arrive at variable rates after compression, depending on the content of the signal. Moreover, it is necessary to track the components of each word that originally belonged to each respective signal.
Several methods may be used to overcome this difference in rates at which the data streams arrive. A first-in, first-out buffer could be used, for example, on the chip used to conduct the DMA transfer. In this case, data stream 1 is transferred until the buffer is full or until all data has been transferred to the buffer from stream 1. If any space is left in the buffer, data stream 2 is then transferred until the buffer is full. Otherwise, the buffer is emptied before data stream 2 can begin transfer. Such a buffer may require a significant amount of memory to accommodate large differences in compression rates.
Alternatively, both bytes from each data stream may be transferred and stored as soon as either of the bytes is full. Each time either data stream 1 or data stream 2 produces a byte's worth of data, the data from both streams is transferred. This sometimes results in one full byte's worth of data and another byte which is not full, which is not efficient. Furthermore, the resources required to transfer two bytes of data are still being used even though less than two bytes are being transferred.
Another option is to transfer each byte from each data stream separately. For example, data stream 1 is transferred to a DMA unit from one “end” of the unit and data stream 2 is transferred to the same unit from another “end” of the unit until the two streams meet, not necessarily in the middle. This takes up two times the resources (e.g. DMA units or processor time) required to effect a transfer and also consumes more current due to more data bus traffic.
Thus, a need exists in the medical arts for transferring and storing data in an implantable medical device.
Several methods have been proposed in the prior art for improving storage and compression in an implantable medical device.
For example, U.S. Pat. No. 5,603,331 to Heemels et al., entitled “Data Logging System For Implantable Cardiac Device” discloses the compression of heart rate variability data via logarithmic data compression and the storing of the results as time-related histograms with a standard deviation.
U.S. Pat. No. 5,819,740 to Muhlenberg entitled “System and Method for Compressing Digitalized Signals in Implantable and Battery-Powered Devices” discloses the compression of data using non-linear sampling. A time varying threshold is used and the signal of interest is compared to the threshold.
U.S. Pat. No. 5,836,982 to Muhlenberg et al., entitled “System and Method of Data Compression and Non-Linear Sampling from Implantable and Battery-Powered Devices” discloses compressing a data block by storing the change, or delta, from one sample to another sample.
U.S. Pat. No. 5,312,446 to Holschbach et al., entitled “Compressed Storage of Data in Cardiac Peacemakers” discloses compression of data using an analog implementation of a turning point algorithm.
U.S. Pat. No. 5,623,935 to Faisandier entitled “Data Compression Methods and Apparatus for Use with Physiological Data” discloses compression of data by generating the first and second derivatives of an analog signal. The first and second derivatives of an analog signal are generated and one of three modes of encoding is selected. Either one of the derivative values is then encoded using one of the three modes based upon maximum compression.
U.S. Pat. No. 5,709,216 to Woodson entitled “Data Reduction of Sensed Values in an Implantable Medical Device Through the Use of a Variable Resolution Technique” discloses compression of data using variable resolution. The variable resolution is based upon pre-selected sub-ranges, i.e., smaller values or intervals have finer resolutions.
U.S. Pat. No. 5,215,098 to Steinhause et al., entitled “Data Compression of Cardiac Electrical Signals Using Scanning Correlation and Temporal Data Compression” discloses data compression by storing pre-recorded (i.e. learned) signal templates.
U.S. Pat. No. 5,217,021 to Steinhause et al., entitled “Detection of Cardiac Arrhythmias Using Correlation of a Cardiac Electrical Signal and Temporal Data Compression” also discloses data compression using stored pre-recorded signal templates.
U.S. Pat. No. 5,836,889 to Wyborny et al., entitled “Method and Apparatus for Storing Signals in an Implantable Medical Device” discloses compression of data for storing a straight-line connection between the last stored value and new data. Data is stored when the first derivative exceeds a threshold.
U.S. Pat. No. 4,716,903 to Hanson et al., entitled “Storage in a Pacemaker Memory” discloses data compression by storing the time to the next sample. The time is stored when the samples are near the baseline. An additional flag is added for turning points.
U.S. Pat. No. 5,263,486 to Jeffreys entitled “Apparatus and Method for Electrocardiogram Data Compression” discloses data compression by varying the sampling period dynamically. The variation is based upon signal rate of change value.
U.S. Pat. No. 4,920,489 to Hubelbank et al., entitled “Apparatus and Method for Solid State Storage of Episodic Signals” discloses compression of data by storing the derivative value, which is defined as data differing from the last stored value. The resolution is also changed based upon the magnitude of rate change.
U.S. Pat. No. 5,735,285 to Albert et al., entitled “Method and Hand-Held Apparatus for Demodulating and Viewing Frequency Modulated Biomedical Signals” discloses transmission of data using A-Law encoding and decoding.
U.S. Pat. No. 5,694,356 to Wong et al., entitled “High Resolution Analog Storage EPROM and Flash EPROM” discloses compression of a signal using A-Law or U-Law log arrhythmic relationships.
As discussed above, the most pertinent prior art patents are shown in the following table:
TABLE 1Prior Art Patents.Patent No.DateInventor(s)5,836,982Nov. 17, 1998Muhlenberg et al.5,836,889Nov. 17, 1998Wyborney et al.5,819,740Oct. 13, 1998Muhlenberg5,735,285Apr. 7, 1998Albert et al.5,709,216Jan. 20, 1998Woodson, III5,694,356Dec. 2, 1997Wang et al.5,623,935Apr. 29, 1997Faisandier5,603,331Feb. 18, 1997Heemels et al.5,312,446May 17, 1994Holschbach et al.5,263,486Nov. 23, 1993Jeffreys5,217,021Jun. 8, 1993Steinhaus et al.5,215,098Jun. 1, 1993Steinhaus et al.4,920,489Apr. 24, 1990Hubelbank et al.4,716,903Jan. 5, 1988Hansen et al.
All the patents listed in Table 1 are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Preferred Embodiments and the claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.